In the manufacture of plastic containers, such as monolayer or multilayer PET containers, it is conventional to mold a container preform having a body and a finish with one or more external threads. The finish typically is molded to its final geometry in the preform, while the body of the preform is subsequently blow molded to the desired geometry of the container body. Although this manufacturing technique is satisfactory for fabrication of containers with narrow-neck finish diameters, the throughput of the process is greatly reduced when employed for fabricating preforms and containers of larger finish diameters. For example, a preform mold cavity block that has ninety-six mold cavities for preforms with a 28 mm finish diameter would typically accommodate only forty-eight cavities having a 43 mm finish diameter for the same overall cavity block size. This throughput is even further reduced for wide-mouth preforms having a finish diameter greater than about 2.0 inches or 50 mm.
To address this manufacturing throughput problem, it has been proposed to fabricate a wide-mouth container by molding a narrow-neck preform and then blow molding the preform body within a cavity that forms the container body, a container finish with external threads, and a trim dome or moil. The trim moil must be removed from the container body and finish, along with the preform finish, after the container is removed from the blow mold. A problem with this technique is that the external threads on the container finish are blow molded, and are not as sharply defined and detailed as desired. Furthermore, this thread definition problem is exacerbated in the case of containers having at least an external surface of polyester construction, such as PET, because the polyester material can strain harden as it expands during blow molding, making it more difficult to force the finish material into the thread-forming portions of the blow mold. It is therefore a general object of the present invention to provide a method of making a plastic wide-mouth container, and/or a plastic wide-mouth container formed by such method, in which the container threads are formed in a post-molding operation so as to possess improved quality and definition.
In accordance with one aspect of the present invention, a method of making a plastic container having a threaded wide-mouth finish includes providing a preform of molded plastic construction. The preform is placed in a mold cavity having a first portion for forming a container body, a second portion adjacent to the first portion for forming a cylindrical finish wall without threads, and a third portion adjacent to the second portion for forming a trim dome or moil. The preform is expanded within the mold cavity to form a one-piece intermediate container product that includes a body, a cylindrical wide-mouth finish wall without threads, and a trim moil. After removing the intermediate product from the cavity, the moil is severed from the end of the finish wall. At least one internal or external thread is then formed on the finish wall by engaging the finish wall with at least one thread-forming tool. In the preferred method of the invention, the cylindrical finish wall is exteriorly engaged by at least one first tool having at least one groove segment. The cylindrical finish wall is simultaneously interiorly engaged by at least one second tool having at least one rib segment, complimentary to the at least one groove segment, to force portions of the cylindrical finish wall into the groove segment and thereby form at least one internal or external thread. One or both of the first and second tools may be heated to facilitate the forming operation.
A wide-mouth blow molded plastic container in accordance with a second aspect of the present invention includes a hollow body and a cylindrical wide-mouth finish integrally blow molded with the body. The finish has at least one internal or external thread swage-formed in the finish wall after molding. At least the exterior surface portion of the finish wall preferably is of PET construction.